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Facile synthesis of size-tunable copper andcopper oxide nanoparticles using reverse

microemulsions

ARTICLE in RSC ADVANCES · FEBRUARY 2013

Impact Factor: 3.84 · DOI: 10.1039/C3RA23455J

CITATIONS

15

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122

4 AUTHORS:

Ajeet Kumar

Clarkson University

41 PUBLICATIONS 761 CITATIONS

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Amit Saxena

University of Delhi


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Arnab De

AbbVie


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Ravi Shankar

Fujifilm Imaging Colorants, Inc.


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Available from: Ajeet Kumar

Retrieved on: 13 March 2016

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Facile synthesis of size-tunable copper and copper oxidenanoparticles using reverse microemulsions

Journal: RSC Advances

Manuscript ID: RA-ART-12-2012-023455.R1

Article Type: Paper

Date Submitted by the Author: 03-Feb-2013

Complete List of Authors: Kumar, Ajeet; University of Delhi, Departmen of ChemistrySaxena, Amit; University of Delhi, Departmen of ChemistryDe, Arnab; Columbia University Medical Center, Department ofMicrobiology and ImmunologyShankar, Ravi; University of Delhi, Department of ChemistryMozumdar, Subho; university of delhi, Chemistry

RSC AdvancesView Article Online

http://dx.doi.org/10.1039/c3ra23455j


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Ajeet Kumar, Amit Saxena, Arnab De*, Ravi Shankar and Subho Mozumdar#

Department of Chemistry, University of Delhi, Delhi-110007, India* Department of Microbiology and Immunology, Columbia University, USATel: +919810728438,

#E-Mail: [email protected]

Graphical Abstract

The synthesis of pure metal and metal-oxide nanoparticles of a desired size remains a significant

challenge. We describe a novel, simple and convenient method for the synthesis of copper and copper oxide

nanoparticles with tailored sizes at room temperature by the reduction of a copper salt (CuSO 4.5H2O) in TX-

100/n-hexanol/cyclohexane/water by a reverse microemulsion route. It was found that reduction with

hydrazine hydrate (reduction potential 1.15V) in an inert N2 environment gives copper nanoparticles whereas

reduction with sodium borohydrate (reduction potential 1.24V) in aerobic condition gives copper oxide

nanoparticles. Several parameters were modulated to examine their effects on the structural properties of

nanoparticles, namely the size and morphology of the nanoparticles. The size of the copper and copper oxide

nanoparticles can be easily controlled by changing the molar ratio of water to surfactant or by altering the

concentration of the reducing agent. The nanoparticles were characterized using a variety of analytical

techniques like X-ray diffraction (XRD), QELS, UV, TEM and EDAX. Our studies reveal that the

Aq. Copper sulfate

TX-100

n-hexanol

Cyclohexane

H2O

Copper Nanoparticles

Copper oxide Nanoparticles

Sodium borohydride

Air atmosphere

Room temperature

Hydrazine hydrate

Nitrogen atmosphereRoom temperature

ge 1 of 21 RSC AdvancesView Article Online



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nanoparticles are spherical in shape and have an average size distribution of 5-100 nm. Our protocol provides a

rapid and low cost procedure for the synthesis of both copper and copper oxide nanoparticles in the same

microemulsion pool. The nanoparticles so formed have been successfully used for catalyzing various chemical

reactions.

Key words: copper nanoparticles; copper oxide nanoparticles; Water-in-Oil Microemulsions;non-ionic surfactant.

Page 2RSC AdvancesView Article Online



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Ajeet Kumar, Amit Saxena, Arnab De*, Ravi Shankar and Subho Mozumdar#

Department of Chemistry, University of Delhi, Delhi-110007, India* Department of Microbiology and Immunology, Columbia University, USATel: +919810728438,

#E-Mail: [email protected]

Abstract

The synthesis of pure metal and metal-oxide nanoparticles of a desired size remains a significant

challenge. We describe a novel, simple and convenient method for the synthesis of copper and copper (II)

oxide nanoparticles with tailored sizes at room temperature from a common copper (II) salt (CuSO4.5H2O) in

TX-100/n-hexanol/cyclohexane/water by a reverse microemulsion route. It was found that reduction with

hydrazine hydrate (reduction potential 1.15V) in an inert N2 environment gives copper nanoparticles whereas

reaction with sodium borohydrate (reduction potential 1.24V) in aerobic condition gives copper (II) oxide

nanoparticles. Several parameters were modulated to examine their effects on the structural properties of

nanoparticles, namely the size and morphology of the nanoparticles. The size of the copper and copper (II)

oxide nanoparticles can be easily controlled by changing the molar ratio of water to surfactant or by altering

the concentration of the reactants. The nanoparticles were characterized using a variety of analytical

techniques like X-ray diffraction (XRD), Quasi Elastic Light Scattering ( QELS), UV-visible absorption

spectroscopy, Transmission Electron Microscopy (TEM) and Energy-dispersive X-ray spectroscopy

(EDAX). Our studies reveal that the nanoparticles are spherical in shape and have an average size distribution

of 5-100 nm. Our protocol provides a rapid and low cost procedure for the synthesis of both copper and copper

(II) oxide nanoparticles in the same microemulsion pool. The nanoparticles so formed have been successfully

used for catalyzing various chemical reactions.

Key words: copper nanoparticles; copper (II) oxide nanoparticles; Water-in-Oil Microemulsions;

non-ionic surfactant.Introduction




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Metal and metal oxide nanoparticles keep attracting the attention of the scientific community because

of their exceptional and unexpected physical and chemical properties coming from quantum confinement at the

nanoscale [1]. Much attention has been paid to these nanoparticles which have different optical, electronic,

magnetic and chemical properties as comparison to their bulk counterparts owing to their exceptionally small

dimensions [2-4]. These small nanoparticles have been found to be seful in the field of catalysis, , as medicines

[5], sensors [6] and infrared sensing materials [7]. Therefore, the development of synthetic routes to obtain

nanoparticles with a controlled shape and a specific size distribution is of paramount importance. Synthesis of

nanoparticles still remains a challenging task owing to intrinsic difficulties in controlling the size, shape,

composition and morphology of the synthesized nanoparticles. Water-in-oil (W/O) microemulsions are

promising in preparing nanoparticles as they act as ‘nanoreactors’ where the size, shape and morphology of

nanoparticles can be controlled in a defined manner.

A water-in-oil (w/o) microemulsion is a particularly attractive reaction medium for preparing metal

nanoparticles [8-10]. These microemulsions consist of nanosized water droplets that are dispersed in a

continuous oil medium and stabilized by surfactant molecules accumulated at the oil /water interface. The

highly dispersed water pools have been shown to be an ideal nano-structured reaction media or microreactor.

This could potentially be used to produceultrafine, monodisperse nanoparticles with a specific shape and

size[11].

Unlike gold and silver, it is difficult to obtain the light transition copper metal by reduction of simple

copper ions in aqueous solution unless other reagents like protective polymers carrying functional groups that

can form complexes with the copper ions are present [12]. However, the microenvironment of the water pools

in w/o microemulsions is significantly different from that of bulk aqueous solution. Hence, we explored the

possibility of synthesizing the copper and copper (II) oxides nanoparticles by reduction of simple copper ion

salts in microemulsions. In this paper we disclose our findings for the synthesis of highly stable, monodisperse,

spherical copper and copper (II) oxide nanoparticles from commonly available CuSO4.5H2O. We discover that

the use of a stronger reducing agent in nitrogen atmosphere (hydrazine hydrate; reduction potential 1.15V)

leads to the formation of copper nanoparticles, while the use of alkaline sodium borohydrate (reduction




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potential 1.24V) in aerobic conditions leads to the formation of copper (II) oxide nanoparticles. Additionally,

the size of the copper and copper (II) oxide nanoparticles can be conveniently controlled either by changing the

molar ratio of water to surfactant or by altering the concentration of the reducing agent.

The stabilising and protective nature of the surfactant molecules reduces agglomeration and shields

the nanoparticles from inadvertent oxidation in case of the metallic nanoparticles. The other advantage of our

method is that it can be prepared at room temperature, unlike other reported methods which requires elevated

temperatures.

2. Experimental

Materials

TX-100 (laboratory grade) was purchased from SRL (India), cyclohexane (99%) and n-hexanol (98%)

were purchased from Spectrochem (India), copper (II) sulfate pentahydrate (98%), hydrazine hydrate (99%)

were purchased from S.D. Fine Chemicals (India), absolute ethanol (99.5%) was purchased from Merck

(Germany). All the chemicals were used without further purification. Double distilled water was used in the

experiments.

Preparation of Copper and Copper (II) Oxide Nanoparticles

A representative synthesis of the copper nanoparticles involved the mixing of two reverse

microemulsions (RM-A and RM-B). RM-A was prepared by taking 25 mL of 0.1 M solution of TX-100 in

cyclohexane and adding 300 µL of n-hexanol and 225 µL of a 5% (w/v) aq. solution of CuSO4.5H2O.

Similarly, RM-B was prepared by taking 25 mL of 0.1 M solution of TX-100 in cyclohexane and adding 300

µL of n-hexanol and 225 µL of a 5% (w/v) aq. solution of N2H4.H20. The typical WO value of this system was

found to be 5. Both the microemulsions were left stirring for 30 minutes so as to obtain an optically clear

homogeneous dispersion. RM-B was then added to RM-A in a drop-wise manner with continuous stirring. The

resulting solution was left for stirring for another 3 hours to allow complete particle growth via ostwald

ripening. Instant development of brown colour indicated the formation of the copper nanoparticles. Nitrogen

atmosphere was maintained throughout the procedure to ensure the complete removal of oxygen and




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preventing the oxidation of the metal [Scheme 1 and Scheme 2]. Copper (II) oxide nanoparticles were

synthesized using a similar procedure. However, 5% (w/v) alkaline.NaBH4 solution was used instead in

aerobic condition [Scheme 1 and Scheme2]. The preparation of copper and copper (II) oxide nanoparticles

using reverse microemulsion is represented in Scheme. 3. Images of copper and copper (II) oxide

nanoparticles prepared by reverse microemulsion at different time intervals are shown in Fig.1. The reaction

was carried out at room temperature. The absorption spectrum of aqueous copper salt, copper nanoparticles

and copper (II) oxide nanoparticles were recorded 3 hours after mixing with a Hitachi AU-2700

spectrophotometer (Fig.2). At the same time, a drop of the colloidal solution was dropped onto a Formvar-

covered Copper grid placed on filter paper and evaporated in air at ambient temperature. Electron micrographs

were taken with a TEM Technai 300KV, ultra twin FEI with EDAX transmission electron microscope

operating at 300 kV. The average particle diameter of the prepared nanoparticles was analyzed by dynamic

light scattering Instrument (Photocor FC, USA). The measuring range was on the scale of 1nm to 5000 nm and

the light source was He-Ne 633 nm laser diode of 1- 40 MW. Data analysis was performed with Alango dynal

V 2.0 software. Wide angle X-ray diffraction pattern were obtained for copper nanoparticles and copper (II)

oxide nanoparticles by using Philips analytica PW 1830 X-ray VB equipped with a 2θ compensating slit,

CuK α radiation (1.54Å) at 40 kV, 40 mA passing through Ni filter with a wavelength of 0.154 nm at 20 mA

and 35 kV. Data collection was made in a continuous scan mode with a step size of 0.01° and step time of 1

sec over a 2θ range of 0° to 120°. Data analysis was performed with PC-APD diffraction software.

3. Result and Discussions

3.1 Effect of Reducing Agent on the Synthesized Nanoparticles

Reaction of copper (II) sulfate pentahydrate with hydrazine hydrate within the aqueous core of the

reverse micellar solution gave a brown colored optically clear solution. The system after processing provided a

brown colored dry powder which could be dispersed and preserved in ethanol so to prevent it from getting

oxidised. In the TX-100 microemulsion system (Wo = 5), the water is tightly bound to the oxyethylene groups

of the polar chain of the surfactant. This leads to a much higher local concentration of copper in water pools of




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w/o microemulsions. Thus, the formation of pure copper metallic particles is favoured. It was found that the

diameter of the reverse microemulsion droplet (water/TX-100/n-hexanol/cyclohexane) increased with the

increasing water content. With this increasing water content and (hence water core volume), the size of the

particles also increased. The inert N2 atmosphere played a significant role. Interestingly, in the absence of N 2

the formed particles were very small (


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the reduction rate of copper (II) sulfate was slow and only a few nuclei were formed at the early period of the

reduction. The atoms formed at the later period collided with the nuclei already formed and contributed more

to the growth of the nanoparticles leading to the formation of larger particles. With the increasing

concentration of the reducing agent, the enhanced reduction rate favoured the generation of many more nuclei.

This would lead to smaller nanoparticles. However when the concentration of the reducing agent was above

0.5M, the copper (II) sulfate was rapidly reduced to unstable nuclei of copper ions. Hence, beyond this

concentration the nucleation rate was not raised and the number of nuclei remained constant with the increase

of hydrazine concentration.

We also studied the size of the nanoparticles with respect to the initial concentration of the metal salt

(copper (II) sulfate). The average diameter of copper and copper (II) oxide nanoparticles was not affected

below a copper (II) sulfate concentration of 0.1 M. When the concentration of copper (II) sulfate was above 0.1

M, the average diameters of the copper nanoparticles increased significantly.

This could be because of two reasons. One was that the concentration of the reducing agent was

relatively low as compared to that of the metal salt and this led to the formation of fewer nuclei at the initial

phase of the reduction. The other was that the number of atoms formed at the very beginning of the reduction

remained constant due to high metal ion concentration. The atoms formed at the latter period contributed to the

growth of the particles and resulted in the formation of larger particles.

The particles of metallic copper did not show any signs of oxidation when nitrogen (N 2) atmosphere

was maintained and hydrazine hydrate was used as the reducing agent. Copper (II) oxide nanoparticles were

obtained when sodium borohydride was used in the absence of nitrogen (N2) atmosphere. The UV–vis

absorption spectrum of copper and copper (II) oxide nanoparticles is shown in Fig. 2. Copper nanoparticles

displayed an optical absorption band at 547 nm. XRD patterns of the copper and copper (II) oxide

nanoparticles prepared are displayed in Fig.5 with 2θ values between 30o

to 90o

. The XRD spectra of copper

nanoparticles Fig 5a shows three characteristic peaks for 2θ at 44.7o, 51.6o, and 76.4o for the respectively

marked indices of (111), (200) and (220).These characteristic peaks confirm the formation of a face-centred

cubic (FCC) copper phase without significant oxides or other impurity phases. Copper (II) oxide synthesised




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using sodium borohydride (Fig 5b) shows its characteristic peaks at 2θ values of t 32.5o, 35.5o, 38.7o,48.6o,

53.5o, 58.2o, 61.4o, 65.8o, 67.8o for the respectively marked indices of (110), (002), (111), (202), (020), (202),

(113), (022), (113) respectively. The average primary particle size of the copper and copper (II) oxide

nanoparticles was calculated from the full width at half maximum (FWHM) of the (111) peaks in the XRD

patterns using the Scherrer equation. This resulted in an average primary particle size of about 34 nm and 41

nm respectively (the larger size of the oxide nanoparticles can also be gauged from the TEM micrograph

images).

The electron diffraction pattern for the resultant nanoparticles as shown in (Fig. 6a) indicated three

main fringe patterns with their radii in the ratio of 1.732 : 2.82 :3.31 A 0 . These relate to the (111), (220), and

(311) planes and revealed that the resultant particles were pure metallic copper with a face-cantered cubic (fcc)

structure. The electron diffraction pattern (Fig. 6b) shows sharp rings with plane distances of 2.99, 2.47, 2.12,

1.51, and 1.28 A°, which match with the d spacing for pure cubic copper (II) oxide. The electron diffraction

pattern for copper (II) oxide nanoparticles resembles the reported pattern as shown by other research groups

[13-15].

For further confirmation, the EDAX was performed for synthesized copper and copper (II) oxide

nanoparticles. The EDAX spectrum given in Fig. 7a shows the presence of copper as the only elementary

component. Fig. 7b shows the oxygen with copper as elementary component which confirms the formation of

copper (II) oxide nanoparticles.

4 . Conclusion

The present study demonstrates a novel, simple and convenient method for the synthesis of copper and

copper (II) oxide nanoparticles from a copper salt (CuSO4.5H2O) in TX-100/n-hexanol/cyclohexane/water in

the reverse microemulsion system. Reduction with hydrazine hydrate in an inert N2 environment gives copper

nanoparticles whereas reaction with sodium borohydride in aerobic condition gives copper (II) oxide

nanoparticles. The reactions are carried out at room temperature. The size of the copper and copper (II) oxide

nanoparticles can be easily controlled by changing the molar ratio of water to surfactant or by altering the




13/24

concentration of the reactants. The prepared nanoparticles were characterized using TEM and QELS which

shows a narrow size distribution. XRD was used and it indicated the cubic phase of spherical copper and

copper (II) oxide nanoparticles which was further supported by the electron diffraction data. Moreover, EDAX

revealed the presence of both elemental Copper and oxygen in the copper (II) oxide nanoparticles. This is a

simple and efficient protocol which provides a rapid and low cost procedure for the synthesis of both copper

and copper (II) oxide nanoparticles in the same microemulsion pool. This is especially important as copper

nanoparticles have been used in the past for catalyzing various chemical reactions such as the chemo-selective

Ullmann coupling [16], synthesis of TZD derivatives [17], cyclisation of Schiff’s base[18], Aza-michael

reaction[19], ligand free C-arylation [20], synthesis of napthoxazinones [21], Husigen cycloaddition reaction

[22] and Biginelli Reaction [23,24]. We have synthesized and used nanoparticles for various other applications

as well [25-31]. Preparation of other nanoparticles is ongoing in our laboratory and the results will be disclosed

in additional publications soon.

Acknowledgments

Authors greatly acknowledges the financial support from CSIR,DST and UGC, Government of India.

Reference

[1]. D. Dodoo-Arhin, M. Leoni, P. Scardi, E. Garnier and A. Mittiga, Materials Chemistry and

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[2]. T.Itakura, K.Torigoe and K. Esumi, Langmuir,1995, 11, 4129.

[3]. L.L. Beecroft and C.K. Ober, Chem. Mater.1997, 9,1302.

[4]. N. Toshima, T. Yonezawa and New J. Chem. 1998, 1179.

[5].

G. J. Brewer, Current Opinion in Chemical Biology, 2003,7, 207.

[6]. S. Sharma, C. Nirkhe, S. Pethkar and A. A. Athawale, Sensors and Actuators B: Chemical, 2002,

85,131-136.




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[7]. P.Donald, Butler, Z.Ç. Butler and R. Sobolewski,Handbook of Advanced Electronic and

Photonic Materials and Devices, 2001,169.

[8]. C.Petit, P.Lixon and M. P. Pileni, J. Phys. Chem.1993 97,12974.

[9].

P.Barnickel, A.Wokaun,W.Sager and H.F.Eicke, J. ColloidInterface Sci. 1992,148, 80.

[10]. P.Barnickel and A.Wokaun, Mol. Phys. 1990,69, 1.

[11]. M. P.Pileni, J. Phys. Chem.1993, 97, 6961.

[12]. H.Hirai, H.Wakabayashi and M. Komiyama, Bull. Chem. Soc. Jpn.1986, 59, 367.

[13]. H.Q. Wu , X.W. Wei, M.W. Shao, J. S. Gu and M. Z.Qu, Chemical Physics Letters,2002, 364,152.

[14]. C.H. Lo, T.T. Tsung, L.C. Chen, C.H. Su and Hong-Ming Lin, Journal of Nanoparticle Research,

2005, 7, 313.

[15]. M. R. Kim, S. J. Kim and D.J. Jang, Crystal Growth & Design,2010, 10, 2010.

[16]. M. Kidwai, N. K. Mishra, V. Bansal, A. Kumar and Subho Mozumdar, Tetrahedron Letters,2007,

48, 8883.

[17]. Kumar, P. Singh, A. Saxena, A. De, R. Chandra and S. Mozumdar, Catalysis Communications,

2008, 17.

[18]. M. Kidwai, V. Bansal, A. Saxena, S. Aerry and S. Mozumdar, Tetrahedron Letters, 2006, 47, 8049

[19].

K. Verma, R. Kumar, P. Chaudhary, A. Saxena, R. Shankar, S. Mozumdar and Ramesh Chandra,

Tetrahedron Letters, 2005, 46, 5229.

[20]. M. Kidwai, S. Bhardwaj and R. Poddar, Beilstein Journal of Organic Chemistry, 2010, 6, 35.

[21]. A Kumar, A Saxena, M Dewan, A De, S Mozumdar - Tetrahedron Letters, 2011, 52 (38), 4835

[22]. A Kumar, S Aerry, A Saxena, A De, S Mozumdar, Green Chemistry, 2012, 14, 1298

[23]. M Dewan, A Kumar, A Saxena, A De, S Mozumdar, PloS One, 2012, 7(1): e29131

[24].

M Dewan, A Kumar, A Saxena, A De, S Mozumdar, PloS One, 2012, 7(8): e43078

[25]. A Kumar, S Kumar, A Saxena, A De and S Mozumdar, Catalysis Communications,2008, 9(5), 778.

[26]. A Kumar, M Dewan, A Saxena, A De and S Mozumdar, Catalysis Communications,2010, 11, 679

[27]. A Kumar, S Kumar, A Saxena, A De and S Mozumdar, Catalysis Letters, 2008, 122, 98




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[28]. M Dewan, A Kumar, A Saxena, A De and S Mozumdar, Tet. Lett, 2010, 51, 6108

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2012, DOI:10.1080/17518253.2012.737029

List of Schemes

Scheme 1: Synthesis of nanoparticles

Scheme 2: Synthesis of copper and copper (II) oxide nanoparticles

Scheme 3: Representation of Preparation of nanoparticles through the reverse microemulsion

method

List of Figures

Fig.1. Photo images of nanoparticles prepared in reverse microemulsion[A] (aq.) Copper (II)

sulfate Pentahydrate [B]Copper nanoparticles after 5 min of mixing RM-A and RM-B [C]

Copper nanoparticles after 3hr of mixing RM-A and RM-B [D] Copper (II) oxide nanoparticles

after 5 min of mixing RM-A and RM-B [E] Copper (II) oxide nanoparticles after 2hr of mixingRM-A and RM-B [F] Copper (II) oxide nanoparticles after 3 hr of mixing RM-A and RM-B

Fig 2 UV absorption spectra of synthesized nanoparticles

Fig 3 TEM micrographs showing the particle size distribution of (a) spherical copper

nanoparticles and (b) copper (II) oxide nanoparticles.

Fig.4

Fig.5 XRD pattern of Copper (a) and (b) Copper (II) oxide nanoparticles.

Fig.6 Electron diffraction pattern of Copper (a) and (b) Copper (II) oxide nanoparticles.

Fig.7 Energy-dispersive X-ray spectroscopy (EDAX) of (a) Copper and (b) Copper (II) oxide

nanoparticles




16/24




17/24

Scheme 1 Synthesis of nanoparticles

Scheme 2 Synthesis of copper and copper oxide nanoparticles

2Cu2+ + NaBH4 + 2H2O 2Cu2O + NaBO2 + 2H2O

2Cu2+ + N2H4 + 4OH- 2Cu + N2 + 4H2O

Aq. Copper sulfateTX-100

n-hexanol

Cyclohexane

H2O

Copper Nanoparticles

Copper oxide Nanoparticles

Sodium borohydride

Air atmosphere

Room temperature

Hydrazine hydrate

Nitrogen atmosphere

Room temperature




18/24

Scheme 3: Representation of Preparation of nanoparticles through the reversemicroemulsion method

100//

4.52 (.)

100//

() (.)

15

15

2

()

5%.4()

, &

, &




19/24

Fig.1. Photo images of nanoparticles prepared in reverse microemulsion[A] (aq.)

Coppersulfate Pentahydrate [B]Copper nanoparticles after 5 min of mixing RM-A and RM-B

[C] Copper nanoparticles after 3hr of mixing RM-A and RM-B [D] Copper oxide

nanoparticles after 5 min of mixing RM-A and RM-B [E] Copper oxide nanoparticles after

2hr of mixing RM-A and RM-B [F] Copper oxide nanoparticles after 3 hr of mixing RM-A

and RM-B




20/24

Fig 2 UV absorption spectra of synthesized nanoparticles




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Fig 3:

() ()




22/24

Fig 4:

() ()




23/24

Fig.5 XRD pattern of Copper (a) and (b) Copper oxide nanoparticles.

Fig.6 Electron diffraction pattern of Copper (a) and (b) Copper oxide nanoparticles.

()

() ()

() ()




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Fig.7 Energy-dispersive X-ray spectroscopy (EDAX) of (a) Copper and (b) Copper Oxide

nanoparticles

()

Weight % Atomic %

CuK 100 100Total 100 100

()

Weight % Atomic %

O K 31.2 64.3

CuK 68.8 35.7

Total 100 100


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